Validating the Effectiveness of Multi-point Wireless Energy Transmission with Carrier Shift Diversity

Recently, Wireless Sensor Networks (WSNs) are used for many applications, e.g. health monitoring of buildings, factory automation, and energy management systems. For example, in Building Energy Management System (BEMS), numerous sensors are deployed in fields such as office room to observe environmental information, e.g. temperature, brightness, human detection, and so on. The information are gathered via wireless links and employed for saving power consumption. However limited life-time of sensors has long been an issue in WSNs. Conventionally, sensor’s energy is supplied by electrical plug, battery or environmental energy, which respectively have the following disadvantages: limitation of installation placement, requirement of battery replacement, or lack of stability. To deal with the problem, we propose to introduce wireless energy transmission to WSNs, in which sensors can be released from both wires and batteries and be stably supplied with energy. Wireless energy transmission schemes are categorized into three types, i.e. radio wave emission, resonant coupling and inductive coupling [1]. In the radio wave emission method, energy is collected at the receiver (Rx) using rectenna (rectifying antenna) to receive and convert radio wave into direct current. Compared with the other schemes, long range transmission can be realized by increasing transmit power or antenna gain. Space Solar Power Satellite (SSPS) is one of the examples of this scheme. In WSNs, numerous ubiquitous sensors are distributed in indoor environments. In order to supply energy to all of them, the radio wave emission is employed in this paper. So far, radio wave emission technology has been mainly used for Radio Frequency IDentification (RFID) systems. However, RFID systems have not been designed for the wide-area coverage targeted in this paper. Additionally, in passive RFID systems in which sensors do not have external battery, radio wave is emitted only when Reader/Writers (R/Ws) request tag data. Consequently, sensors cannot transmit the data on their own initiative. Furthermore, in a single transmitter (Tx) system, the coverage of energy supply field is restricted by maximum transmit power limited by the radio regulation. To extend the coverage, multiple transmitters can be introduced to the systems. This, however, results in the collision between multiple transmitters. To avoid the collision while complying with the regulation, a collision avoidance scheme among multiple R/Ws, e.g. Time Division